An in vitro assay for muscle proteolysis has been developed. Test sera from normal, septic, and cachectic animals have demonstrated that septic and cachectic animals' sera carry a factor or factors which increases muscle proteolysis. Sarcoma patients have been studied with intravenous glucose tolerance tests and have been shown to be glucose intolerant (1). Exogenous insulin has been administered to rats bearing sarcomas. Insulin given to tumor bearing animals increases spontaneous food intake, nitrogen balance, body weight gain, and does not promote tumor growth. However, survival studies show no advantage to the insulin treated animals. (2). We have characterized the effects of tumor mass by creating an artificial tumor model that simulates the mass characteristics of our fisher rat sarcoma tumor system (3,4). We know that cachexia is sarcoma dependent in our animal system, because if we remove the tumor, we can totally reverse the cachexia. We have data in parabiotic animal tumor systems that indicates that the sarcoma secretes a factor which mediates the anorexia and body weight loss seen in cachexia. If experiments continue to show that a tumor factor mediates metabolic derangements, we will attempt to describe and isolate the factor.